A robot comprising more than one arm and where at least two arms each form chains of joints between the actuators of the robot and the platform that is to be manipulated is called a parallel-arm robot or a parallel-kinematic robot. For a fully extended parallel-arm robot for movement of a platform with three degrees of freedom (e.g. in directions x, y and z in a Cartesian system of coordinates), three parallel-working arms are required. If all six degrees of freedom of the platform (x, y, z and the tool orientation) are to be manipulated, six parallel-working arms are required. Each such arm comprises an upper arm and a lower arm. In several applications, a manipulation with a combination of degrees of freedom for positioning and degrees of freedom for orientation is desired. One class of such applications is interior work in narrow spaces. In that case, it is often desired to have a robot with two degrees of freedom for tool orientation and only one degree of freedom for radial positioning.
To obtain a rigid arm system with a large loading capacity and a low weight, the lower arms of the parallel-kinematic manipulator, nearest the manipulated platform, shall consist of six links in total, which only transfer compressive and tensile forces. For a manipulator with three degrees of freedom and three arms, this means that these six links must be distributed among the arms and this can only be done with two different combinations 2/2/2 or 3/2/1.
To more readily describe parallel-kinematic robots comprising linkages, some definitions of different linkages are introduced here:                Link: A link is a member that movably joins two elements and that, at each end, allows movement in three degrees of freedom. It usually consists of a rigid elongated member such as, for example, a rod that has a ball joint at each end. The link holds the elements at a definite distance from each other and absorbs only tensile or compressive forces. Thus, a link transfers no torsional movements.        Double link: A double link is a member that movably joins two elements, that at each end allows movement in three degrees of freedom, and that transfers a moment in a plane between the elements. The double link consists of a quadrangle with, for example, two links, according to the above, that form a first pair of links and the elements that form the second pair of links. In a special case, the double link is a parallelogram, in which case the two elements are forced to move in parallel with each other. Since all joints allow movement in three degrees of freedom, this implies that the double link may be twisted. Thus, the double link needs help from other linkages to remain plane.        Locked double link: A locked double link is a member that movably joins two elements that, at each end, allows movement in two degrees of freedom. The locked double link consists of a double link according to the above, wherein at least one diagonal is locked. This is achieved, for example, by introducing in the quadrangle an additional link that is not parallel to any of the other links. This prevents the elements from being displaced, but still the locked double link may be twisted.        Triple link: A triple link is a member that movably joins two elements, that at each end allows movement in three degrees of freedom, and that transfers a moment in two planes between the elements. The triple link usually consists of two double links, oriented in different planes, with one common link. In a special case, the triple link comprises a space parallelogram consisting of three parallel links of equal length. Such a space parallelogram may be twisted but maintains the elements oriented in parallel planes.        Triangle: A triangle is a member that movably joins two elements and that at one end (the base) allows movement in one degree of freedom and at its other end allows movement in three degrees of freedom. The triangle consists, for example, of a torsionally rigid member that, at its base, is journalled to a first element through an axis and at its other end is journalled to a second element by means of a ball joint. A triangle may also consist of two links according to the above, where one of the joints is common.        
In the following text, an arm for a robot shall mean a linkage supported by a supporting arm. By the concept supporting arm is to be understood a torsionally rigid member that movably joins two elements together and that, at both ends, allow movement in one degree of freedom. The supporting arm consists, for example, of a tube with a fork arranged at each end through which an axle passes. In a special case, the axles are parallel whereby the elements joined by the supporting arm are allowed movement in one plane only. It should be mentioned that the movement comprises rotation as well as translation. The supporting arm may, from one element to the other, transfer both tensile and compressive forces, torsional moment and bending moment.
When a working range in the form of a sphere or an ellipsoid is required, serial kinematics is currently used, wherein the first three joints (axes) are substantially responsible for the positioning of the tool and the other axes (nearest the tool) are substantially responsible for the orientation of the tool. The most commonly used kinematics is thus the following: Axis 1 rotates the whole robot structure, axis 2 rotates a first arm in the vertical plane, axis 3 rotates a second arm, also in the vertical plane, axis 4 rotates the tool around an axis in the centre of the second arm, axis 5 rotates the tool around an axis perpendicular to axis 4, and axis 6 rotates the tool around an axis perpendicular to axis 5. This serial construction of the robot results in the following problems:                The robot becomes heavy, which limits its speed and which necessitates expensive and energy-consuming actuators (motors). If, in addition, the robot is to be mounted as a slave on a master manipulator in order, for example, to guide the robot into narrow spaces, a heavy slave-type robot gives a still heavier and more expensive master manipulator.        The robot becomes pliant and when objects or tools are moved, a undesired oscillating motion is obtained along the path along which the movement is to be made, and when the movement is to be stopped, so-called overshoots arise. This problem is increased further in case of a master/slave configuration, whereby also the performance of the master manipulator is deteriorated.        The robot becomes resilient when forces are to be generated between the tool and the object, which is impaired in a master/slave configuration.        For the serially mounted movable actuators with associated measurement sensors, movable cabling is required, which reduces the reliability of the serially kinematic robots.        It is difficult to obtain a high accuracy of the robot without providing very expensive solutions with precision components and precision assembly of the robot.        Because of temperature-dependent arm lengths and geometrical relations of the serially constructed robot, the tool positioning becomes temperature-dependent.        
All of these limitations with currently used serial kinematic robots can be eliminated with a parallel-kinematic robot that is driven by parallel-working linkages, which do not need to support one another.
One example of such a robot is previously known from the patent publication WO99/58301. This robot has, in its basic design, three degrees of freedom and comprises three linkages that are connected in parallel between three rotating actuators and one manipulated platform. Each linkage consists of a pivoting arm, which via two parallel-working articulated rods is connected to the manipulated platform. At both ends of the articulated rods, joints with two or three degrees of freedom are positioned. The parallel kinematic robot thus obtained has a toroidal working range around a base platform, on which the actuators are mounted. The manipulated platform will not change its angle of inclination during the positioning and the extent of the working range above and below the robot is limited by the parallel articulated rods between the lower arms and the manipulated platform. For this reason, the robot cannot be designed for a working range in the form of a sphere or an ellipsoid.
A parallel-kinematic manipulator that is capable of changing the inclination of the manipulated platform and that has a possibility of providing a working range in the form of a sphere or an ellipsoid is described in U.S. patent specification U.S. Pat. No. 4,651,589. Two elements are here connected to at least three legs that are placed in separate planes. The legs each consist of two parts, each part being connected at one end to the other part of the same leg by a spherical connection and at the other end to one of the two elements, however not the element to which the other part of the same leg is connected, by a rotational connection. However, this structure provides a working range that is barely half an ellipsoid and that is therefore not sufficient to build a robot capable of working in all directions with a working range corresponding to a whole ellipsoid or sphere. To sum up, the manipulator disclosed in U.S. Pat. No. 4,651,589 has the following limitations.                A small working range (maximum half an ellipsoid).        Since the legs are working in different planes, all the axes of rotation of the driving motors will have different directions, which makes it impossible to have two parallel or coinciding motor shafts. However, this is necessary for obtaining a simple motor drive that is not space-demanding, which is especially important when the manipulator is to be used as a wrist with transmissions between, on the one hand, motors at one end of a robot arm and, on the other hand, the wrist at the other end of the same robot arm.        The structure is only intended for manipulators with rotating motors that, via pivoting arms, give the manipulated platform its movements. In several applications, it is desirable to use linear actuators without pivoting arms, which is not possible with legs working in different planes.        
These limitations, which are fundamental for applications in practice, are not present in the parallel-kinematic robot according to the present invention, which is due the following facts:                Two linkages (corresponding to legs in U.S. Pat. No. 4,651,589) are working in a common plane.        These two linkages may therefore, nearest the manipulated platform, each have a set of articulated rods mounted on the manipulated platform with joints where a line through these joints (called lead line) of one set of articulated rods becomes parallel to the corresponding lead line of the other set of articulated rods.        These parallel lead lines then provide a possibility of having two rotating actuators with parallel or coinciding axes of rotation, which via pivoting arms manipulate the two linkages.        This configuration of the motor shafts provides a possibility of obtaining a working range of +/−180 degrees and even +/−200 degrees, which results in a working range of the robot corresponding to a whole sphere or ellipsoid. No previously known parallel-kinematic structure has such a large solid-angle working range.        In the case of coinciding axes of rotation, in addition, the manipulated platform may be rotated an infinite number of turns around the axis of rotation which is then common to the two motors, which makes it possible always to program the robot to take the nearest path between two positions/orientations of the manipulated platform (it is not necessary to rotate back one turn if, for example, one position lies at the end of a turn and the next position lies at the beginning of the same turn).        Furthermore, with coinciding axes of rotation, these may be driven with coaxial tube transmissions, which enables having the driving motors at one end of a robot arm and the axles for driving the two linkages at the other end of the robot arm.        The parallel lead lines for the two linkages then enable the introduction of a third linkage with a set of articulated rods that has a lead line that is perpendicular to the lead lines of the first two sets of articulated rods.        This in turn provides a possibility of introducing a third motor with an axis of rotation that is perpendicular to the axes of rotation of the other two motors. In this way, a single belt transmission or a tube transmission with a bevel gear pair may be used in a wrist for transmitting motor torque from the motor at one end of the robot arm to the third linkage.        The parallel lead lines for the two sets of articulated rods then provide a possibility of introducing, for the set of articulated rods of the third linkage with a lead line that is perpendicular to the lead lines of the first two sets of articulated rods, three articulated rods, whereby a freely rotatable motor for the third linkage will always be controlled by the other two motors such that the third linkage ends up midway between the other two linkages, which gives an optimal kinematic configuration, both with regard to the magnitude of the working range and the rigidity and performance of the robot.        In addition, the parallel lines for the two linkages provide a possibility of mounting the sets of articulated rods with parallel lead lines on linear actuators to obtain an optimum performance in one plane. By then mounting the third set of articulated rods, with a lead line perpendicular to the other lead lines, on a third linear actuator, either directly or via a pivoting arm, optimum performance are also obtained in space between the linear actuators.        
With this parallel-kinematic configuration, a robot is obtained which has all the advantages that are characteristic of a parallel-arm robot with six links to the manipulated platform and which therefore solves the problems for serial arm robots previously described. At the same time, an agility, as great as that of a serial anthropomorphic robot, is imparted to this robot, which implies that it may be used, for example, in narrow spaces. However, the agility enables this robot structure to be utilized in other applications as well. A few examples of areas of applications of parallel-kinematic robots, composed according to the structure described in this patent document, will be given below:                Arc welding, especially in master/slave configurations, for example in ships' modules.        Spot welding, for example in wheel housings.        Gluing, cementing, painting, for example inside engine compartments, boots, fuselages.        Assembly, for example inside car bodies in a master/slave configuration.        Measurement, for example in engine blocks, gearbox casings and car bodies.        Grinding, deburring, milling, boring, etc., for example in engine blocks, gearbox casings, certain casting.        Laser cutting, water cutting, for example in car and aircraft components.        Machining, where the robot is used for inserting and withdrawing objects in various types of machines, for example multi-operation machines, foundry machines, punches and high-pressure presses.        